The invention relates to an A/D conversion device having an input intended to receive an analog input signal, and an output intended to supply a digital output signal, comprising:
an amplifier having an input intended to receive the analog input signal of the device, and an output, PA1 an A/D converter having an analog input connected to the output of the amplifier and an output intended to supply the digital output signal of the device, comprising a resistance ladder traversed by a current supplied by a controllable bias current source, the junction points between said resistances supplying a plurality of reference voltages intended to be compared with the signal received at the input of the converter, the results of these comparisons being used to elaborate the digital output signal of the device. PA1 a comparison module, activated when the calibration signal is active, comprising a comparator intended to supply, at an output, a signal which is representative of at least one comparison between the digital output signal and a predetermined binary word, PA1 a control module having an input connected to the output of the comparison module and being activated when the calibration signal is active, intended to supply a digital control signal, whose value depends on the value of a signal received at the input of the control module, and intended to effect a variation of the value of the current supplied by the bias source, and PA1 storage means in which the value of the control signal is stored when the calibration signal is inactive. PA1 the bias current source comprises N current sources which are parallel arranged between the second power supply terminal and one end of the resistance ladder, the value I.sub.i of the nominal current supplied by the current source of row i (for i=1 to N) being equal to I0/2.sup.(i-1) in which I0 is a predetermined value, PA1 the control signal is a binary word of N+1 bits, each of which, when being in an active state, controls the conduction of one of the N current sources constituting the bias current source. PA1 the calibration signal being active during at least N times a comparison cycle, in the course of which an analog signal at the output of the amplifier is converted into a digital signal and then compared with the predetermined binary word, the comparison module is provided with a register intended to store, in the course of a control cycle whose duration is at least equal to that of a comparison cycle, the value taken by the output signal of the comparator in the course of at least one of the previous control cycles, PA1 the control module is provided with means for systematically placing, in the course of the first control cycle, the bit of rank 1 of the control signal at the active state, PA1 the control module is provided with means for systematically placing at the active state, in the course of any subsequent control cycle, the bit of the rank subsequent to that of the previous bit whose state has been modified in the course of the previous cycle, the previous bit being unchanged if the value of the signal which it receives at its input is identical to that of the signal received at the end of the previous control cycle, and inversed in the opposite case, the process being repeated until the DC level calibration signal resumes the inactive state. PA1 the calibration signal being active during at least N.P comparison cycles, each control cycle comprises P comparison cycles, PA1 the comparison module is provided with a register intended to store the values taken by the output signal of the comparator at the end of each one of the P successive comparisons carried out in the course of a same control cycle, and with a weighting module intended to supply, at an output, a signal whose value is representative of an average of said values, said output constituting the output of the comparison module. PA1 a first A/D conversion device having an input and an output respectively intended to receive, the analog input signal of the circuit and to supply a signal of a digital kind, referred to as most significant word, constituting the most significant part of the digital output signal of the circuit, PA1 a D/A converter intended to receive the most significant word and to convert it into an output signal of an analog kind, referred to as converted signal, PA1 a second A/D conversion device having an analog input intended to receive the difference between the converted signal and the input signal of the circuit, and an output intended to supply a signal of a digital kind, referred to as least significant word, constituting the least significant part of the digital output signal of the circuit, PA1 said conversion circuit being characterized in that at least one of the two A/D conversion devices is in accordance with any one of the embodiments described hereinbefore.
Such a device is described in U.S. Pat. No. 5,313,207. This device is used within an A/D conversion circuit having a two-step structure in which a primitive analog signal is converted into a digital signal by a first A/D converter which supplies, at the output, a digital signal referred to as most significant word which constitutes an approximation of the digital value of the analog input signal and thus forms the most significant part of the digital output signal of the conversion circuit. The most significant word is converted into an analog signal by a D/A converter which supplies, at the output, a signal referred to as converted signal which corresponds to the transposition to an analog value of the approximate digital value supplied by the first A/D converter. Here, this converted signal takes the form of a current which acts on a device as described in the opening paragraph, in which the amplifier receives the primitive analog signal at the input. In this device, the control signal is actually constituted by the most significant word, the operation consisting of adjusting the value of the output voltage of the amplifier via the regulating means incorporating the analog conversion of said word. The amplifier thus supplies, at the output, an analog signal referred to as residual signal which is representative of a quantization error due to the approximation effected by the first A/D converter, which residual signal is converted within the device by a second A/D converter into a digital signal referred to as least significant word which thus forms the least significant part of the digital output signal of the conversion circuit. The gain of the amplifier must be such that to the maximum excursion of its input voltage corresponds the maximum value of the least significant word. If the amplifier has a gain which is too high, the second A/D converter tends to come in a saturation mode, i.e. it supplies the same output signal for all the input signals having a value which exceeds a given threshold it receives. If the gain value is too low, certain values which are available at the output of the second A/D converter will never be reached by the least significant word. These phenomena may cause discontinuities in the development of the value of the output signal of the device as a function of the development of the value of the primitive analog signal, which is not acceptable.